The ability of various doses of sulfinpyrazone and ibuprofen to inhibit prostaglandin-mediated coronary vasodilation and to inhibit platelet aggregation was evaluated in dogs. Doses of these drugs that uniformly inhibited platelets also markedly reduced vasodilator responses to the prostaglandins precursor arachidonic acid. Threshold doses for inhibition of coronary vasodilation and platelet aggregation were also similar. We conclude that doses of sulfinpyrazone or ibuprofen sufficient to interfere with platelet aggregation are also likely to inhibit prostaglandin-mediated coronary vasodilation, possibly through disruption of prostaglandin synthetic pathways in both platelets and vascular endothelium.